Biology · Caffeine · 5 min read
How long does caffeine stay in your body?
About half the caffeine from a 6 pm cup is still in you at 11 pm. The arithmetic, cup by cup, and what the sleep studies found.
In short
- Caffeine's half-life in adults is 3 to 7 hours. About half of a 6 pm cup is still in you at 11 pm.
- In one trial, 400 mg taken 6 hours before bed cut sleep by more than an hour.
- A review of 24 studies puts the last coffee at least 8.8 hours before bedtime.
What half-life means for your cup
Your gut takes in caffeine fast. A 2017 review of caffeine safety says it is "absorbed rapidly and totally in the small intestine in less than 1 h", and the level in your blood peaks after about two hours.1 From there your liver clears it at a steady pace.
The same review puts the half-life at 3 to 7 hours in adults.1 A report from the US Institute of Medicine puts the average near 5 hours, with a range of 1.5 to 9.5.2
Half-life is the time your body takes to clear half of what is in it. On a 5-hour half-life, a cup at 6 pm leaves half its caffeine in you at 11 pm and a quarter at 4 am.
The chai maths
The European Food Safety Authority (EFSA) lists about 50 mg of caffeine in a 220 ml cup of black tea and 90 mg in a 200 ml cup of filter coffee.3 We could not find a measured figure for Indian milk chai, so the table uses EFSA's tea number. A cutting glass holds less than 220 ml and a big mug holds more, so scale the numbers to your cup.
| Last cup at | Hours before an 11 pm bedtime | Caffeine still in you at 11 pm | From a 50 mg tea | From a 90 mg coffee |
|---|---|---|---|---|
| 9 pm | 2 | 76% | 38 mg | 68 mg |
| 7 pm | 4 | 57% | 29 mg | 52 mg |
| 5 pm | 6 | 44% | 22 mg | 39 mg |
| 3 pm | 8 | 33% | 16 mg | 30 mg |
| 1 pm | 10 | 25% | 13 mg | 23 mg |
| 11 am | 12 | 19% | 9 mg | 17 mg |
The table is arithmetic on a 5-hour half-life. It skips the hour or two your body takes to absorb a cup, and your own half-life may sit anywhere between 3 and 7 hours.
Cups add up. Each one follows its own curve, and at bedtime you carry the total.
What it does to your sleep
Caffeine "works by binding to adenosine receptors", and adenosine is a neuromodulator that "can promote sleep".1
A team at Henry Ford Hospital in Detroit gave 12 healthy adults 400 mg of caffeine at bedtime, 3 hours before it, or 6 hours before it, and tracked their sleep at home with a headband monitor.4 They wrote: "Even at 6 hours, caffeine reduced sleep by more than 1 hour." By EFSA's figures 400 mg is more than four cups of filter coffee at once, so treat it as a strong dose.
A 2023 review pooled 24 studies. Caffeine cut total sleep by 45 minutes, added 9 minutes to the time people took to fall asleep, and trimmed deep sleep by about 11 minutes.5 The authors worked out cut-off times from the data: a 107 mg coffee at least 8.8 hours before bed, and a 217.5 mg pre-workout at least 13.2 hours before. For an 11 pm bedtime, that puts the last coffee a little after 2 pm.
Smaller doses count too. EFSA notes that single doses of 100 mg "may affect sleep duration and patterns in some adults, particularly when consumed close to bedtime".3 That is two cups of tea by its figures.
Why the same cup hits people differently
A liver enzyme called CYP1A2 handles about 95% of the caffeine you take in, and how fast it works varies from person to person.6 A few things shift it:
- Smoking doubles the rate at which the body clears caffeine.1
- Oral contraceptives "tend to double the half-life of caffeine".1
- In pregnancy the half-life runs 8.3 hours longer on average.1
Caffeine also interacts with many medicines.6 If you take one every day, ask your doctor or pharmacist how it sits with your tea and coffee.
How much is too much
EFSA's safety review found that up to 400 mg a day, spread through the day, raises no safety concern for healthy adults, and neither does a single dose of up to 200 mg.3 For pregnant and breastfeeding women the figure is 200 mg a day. In EFSA's cup sizes, 400 mg is eight teas or a little over four filter coffees.
Those are safety limits. Sleep reacts to less, as the studies above show.
What to do with this
- Count back from your bedtime. The review's line for coffee is 8.8 hours.5
- Per cup, EFSA's tea figure is a little over half its coffee figure, so an evening chai leaves less in you than an evening coffee. The table shows how much.
- If you are pregnant or on the pill, your half-life is longer than the table assumes. If you take daily medicine, ask your doctor.
References
- Temple JL, Bernard C, Lipshultz SE, Czachor JD, Westphal JA, Mestre MA. The Safety of Ingested Caffeine: A Comprehensive Review. Frontiers in Psychiatry, 2017. pmc.ncbi.nlm.nih.gov/articles/PMC5445139
- Institute of Medicine. Caffeine for the Sustainment of Mental Task Performance, chapter 2, Pharmacology of Caffeine. 2001. ncbi.nlm.nih.gov/books/NBK223808
- European Food Safety Authority. Caffeine, summary of the 2015 Scientific Opinion on the safety of caffeine. efsa.europa.eu/en/topics/topic/caffeine
- Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 2013;9(11):1195-1200. pmc.ncbi.nlm.nih.gov/articles/PMC3805807
- Gardiner C, Weakley J, Burke LM, Roach GD, Sargent C, Maniar N, Townshend A, Halson SL. The effect of caffeine on subsequent sleep: A systematic review and meta-analysis. Sleep Medicine Reviews, 2023;69:101764. pubmed.ncbi.nlm.nih.gov/36870101
- Nehlig A. Interindividual Differences in Caffeine Metabolism and Factors Driving Caffeine Consumption. Pharmacological Reviews, 2018;70(2):384-411. pubmed.ncbi.nlm.nih.gov/29514871